The present invention relates to temperature compensation of time sources, and more particularly relates to compensation of digital clocks.
The frequency source for many clock configurations changes with temperature. This phenomenon can be problematic when an accurate time source is desired. The problem is particularly acute for clock sources based on a piezoelectric crystal. Piezoelectric crystals usually exhibit a nonlinear temperature versus frequency characteristic as disclosed in U.S. Pat. No. 5,200,714 to Hayashi for an AT cut piezoelectric quartz frequency source. U.S. Pat. No. 5,041,799 to Pirez discloses similar nonlinear curves for an AT cut crystal configured for use in an oscillator. In addition, the frequency response of piezoelectric crystals has been known to change with age of the crystal.
One solution to this problem has been to place the crystal and other temperature sensitive components of a crystal-based frequency source in an oven. Typically, the oven maintains a temperature corresponding to a zero slope region of the temperature versus frequency curve for the selected crystal configuration. Unfortunately, this kind of compensation system is usually quite expensive. Also, the space and energy required for an oven may make this approach impractical.
Other attempted solutions include adjustments to the load reactance of a crystal-based oscillator circuit to compensate for changes with temperature and age. These systems often involve a varactor to adjust oscillator load reactance. U.S. Pat. Nos. 5,428,319 to Marvin et al., 5,204,975 to Shigemori, 5,162,758 to Onishi, and 4,302,731 to Ashida disclose various schemes to improve oscillator performance through adjustment of load reactance. The degree of compensation possible for this type of system is generally limited by the initial quality of the crystal and may not be cost effective for some applications. Furthermore, this approach may not be satisfactory for low frequency applications requiring a very high degree of precision.
Thus, there remains a need for a clock compensation system with improved efficiency which may be used to provide a highly precise low frequency time source. Preferably, this need may be satisfied without relying on an oven or control of oscillator load reactance.